Technical Field
Aspects of the embodiments relate to use of current-mode feedback control of a multi-channel Class-D audio amplifier.
Background Art
Typically one of the last components in an audio distribution chain, audio amplifiers amplify a low power audio signal to a level suitable for driving one or more loudspeakers. Multi-channel audio amplifiers are employed throughout structures to amplify more than one channel of audio.
As known to those of skill in the art, typical Class-D audio amplifiers are a class of audio amplifier in which the audio signal modulates a pulse width modulated carrier signal to drive the output. Referring now to FIG. 1, a block diagram of class-D audio amplifier 100 is illustrated showing the different stages of operation of the audio amplifier. Input audio signal (input signal) 102 is input to pulse width modulator (PWM) comparator 104, which is part of modulator block 109, along with triangle wave 105 that has been generated by triangle wave generator 103. In PWM comparator 104, input signal 102 is compared to triangle wave 105. Triangle wave (or sawtooth wave) 105 is typically a much higher frequency signal than input signal 102—usually ten or more times the highest expected frequency of input signal 102. Thus, if an audio signal of about 20 Hz to about 20 kHz is expected as input signal 102, then triangle wave 105 should be at least about 200 kHz. As its name implies, PWM comparator 104 typically includes a comparator, and compares the amplitudes of input signal 102 to that of triangle wave 105 to form a pulse width modulated output. That is, the width of each pulse will be dependent upon the amplitude of both input signal 102 and the amplitude of triangle wave 105, but at a frequency equal to that of triangle wave 105. The duty cycle is proportional to the amplitude of input signal 102. If both input signal 102 and triangle wave 105 are centered about 0 volts, then for a 0 volt input signal 102, the output duty cycle is about 50%, and if input signal 102 is about Vtmax, the maximum of triangle waveform 105, the duty cycle is about 100%. If input signal 102 is about Vtmin, the lowest voltage value of triangle waveform 105, then the duty cycle is about 0%. FIG. 2 illustrates an example of a typical pulse width waveform generated in the above-described manner.
Referring now to FIG. 2, input signal 102 is shown as a roughly sinusoidal shaped signal with a frequency of about 1/20th  of that of triangle waveform 105. Comparator stage output 106 is shown below input signal 102 that is super-imposed over triangle waveform 105; the logic level “high” represents the times in which the amplitude of input signal 102 exceeds that of triangle waveform 105. It can be seen that at points A and B, the duty cycle of comparator stage output 106 (the pulse width modulated signal) is about 50%, and as input signal 102 swings closer to Vtmax (at point C) and Vtmin (at point D), the duty cycle of comparator stage output 106 changes from about 100% to about 0%, respectively.
As known by those of skill in the art, typical Class-D audio amplifiers with multiple channels typically comprise a global triangle ramp generator for use in all of the channels. A global analog buffer and a local analog buffer can be inserted between each channel and the triangle ramp generator.
Following PWM comparator 104, comparator stage output 106 is input to switching output stage (or power stage) 108, which is also part of modulation block 109. This device is typically comprised of an arrangement of switching transistors configured as a “half-bridge” or “full-bridge” and it amplifies the signal input to it, to create switching output signal 110. As can be seen in FIG. 1, switching output signal 110 is an amplified version of comparator stage output 106, and switches between only two states, the positive and negative power supply rail voltages (in the case of the “half-bridge” implementation), and the positive power supply rail voltage and ground (in the case of the “full-bridge” implementation). As known to those of skill in the art, the gain of the typical Class-D audio amplifier modulator stage is set by the ratio of the power supply voltage, (or at least the power supply voltage that is available at switching output stage 108) and the peak-to-peak triangular wave voltage (triangle waveform 105). For example, the gain of the modulator stage, of a half-bridge implementation with +/−50V DC power supply rails, and a triangular wave voltage of 10 Vpp is 100V/10V=10.
Following switching output stage 108 is filter stage 112. In filtering stage 112, the amplified PWM signal is passed through an (ideally) lossless low pass filter (LPF) prior to the output device, speaker 116. The LPF removes the high frequency components of the PWM signal (switching output signal 110) and recovers the original audio signal, but in an amplified form, now referred to as amplified output signal 114.
Having briefly reviewed operation of a Class-D audio amplifier in a fairly general sense, attention can now be directed to specific design issues with regard to Class-D audio amplifiers. For example, it is known to those of skill in the art of power supplies to use current-mode control in buck-derived switch-mode power supplies. The principal benefits of current-mode control are the conversion of the double-pole passive inductor-capacitor (L-C) output filter of the Class-D audio amplifier topology into a single-pole output, greatly simplifying the task of feedback loop design, and achieving a stable control loop with adequate gain and phase margins. This feedback loop design task is especially difficult when the loop is closed at the output of the audio amplifier channel, after the L-C filter. However, it is after the L-C filter where the feedback loop must be closed, if state-of-the-art audio performance is desired. The goals of a state-of-the-art Class-D audio amplifier design involve achieving extremely low total harmonic distortion (THD) and noise levels, while maximizing dynamic range (signal-to-noise ratio (SNR)), frequency response, and flatness (at least over the expected frequency response). Additionally, for a Multi-Channel design, there is the requirement of minimizing channel-to-channel crosstalk. All of these design goals are aided by a feedback loop design utilizing negative feedback to reduce open loop errors. This feedback design needs to maintain a loop gain as high as possible over the audio frequency range (from about 20 Hz to about 20 KHz), and with as high a bandwidth as possible. This is made very difficult by the presence of the double-pole characteristic of the typical L-C output filter that is a part of most high-fidelity Class-D audio amplifier topologies. Current-mode control makes the design of this feedback loop more manageable, as described in greater detail below, as it simplifies the design of the feedback voltage amplifier. That is, a larger amount of passive components (such as resistors and capacitors) would be needed in the design of the feedback voltage amplifier. An additional characteristic, which is desirable for a multi-channel audio amplifier design, is the capability for two independent channels to be connected in a bridge-tied-load (BTL) configuration to drive a single speaker at up to four times the output power of a single channel.
As mentioned herein, the use of current-mode control is known to those of skill in the art of both switching power supplies and Class-D audio amplifiers. In the case of the latter, it is also known to those of skill in the art that current-mode control can be used as part of a double feedback loop system. However, many questions about such use remain unanswered, specifically, exactly how the current-mode control is implemented, whether peak current-mode control or average current-mode control is used, or how, and where, the current sensing signal is detected and processed. Further, there is no known knowledge or evidence from known usages of current-mode control in Class-D audio amplifiers about the suitability of the stated method of double-loop control in regard to multi-channel audio amplifier designs, in which it is desired to connect two independent channels in a BTL configuration.
Thus, there is a need for current-mode control of bridge tied load configurations of Class-D audio amplifiers in order to provide the benefits of both configurations in amplifying audio signals in a cost effective and efficient manner.